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  <title>The design of FreeType 2</title>
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<h1 align=center>
  The design of FreeType&nbsp;2
</h1>

<h3 align=center>
  Copyright&nbsp;1998-2000 David Turner (<a
  href="mailto:david@freetype.org">david@freetype.org</a>)<br>
  Copyright&nbsp;2000, 2007 The FreeType Development Team (<a
  href="mailto:devel@freetype.org">devel@freetype.org</a>)
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    <h1>
      I. Components and APIs
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  <p>It's better to describe FreeType&nbsp;2 as a collection of
  <em>components</em>.  Each one of them is a more or less abstract part of
  the library that is in charge of one specific task.  We will now explicit
  the connections and relationships between them.</p>

  <p>A first brief description of this system of components could be:</p>

  <ul>
    <li>
      <p>Client applications typically call the FreeType&nbsp;2
      <b>high-level API</b>, whose functions are implemented in a single
      component called the <em>Base Layer</em>.</p>
    </li>
    <li>
      <p>Depending on the context or the task, the base layer then calls one
      or more <em>module</em> components to perform the work.  In most
      cases, the client application doesn't need to know which module was
      called.</p>
    </li>
    <li>
      <p>The base layer also contains a set of routines that are used for
      generic things like memory allocation, list processing, i/o stream
      parsing, fixed-point computation, etc.  these functions can also be
      called by a module at any time, and they form what is called the
      <b>low-level base API</b>.</p>
    </li>
  </ul>

  <p>This is illustrated by the following graphics (note that component
  entry points are represented as colored triangles):</p>

  <center>
    <img src="basic-design.png"
         width="394" height="313"
         alt="Basic FreeType design">
  </center>

  <p>Now, a few additional things must be added to complete this
  picture:</p>

  <ul>
    <li>
      <p>Some parts of the base layer can be replaced for specific builds of
      the library, and can thus be considered as components themselves.
      This is the case for the <tt>ftsystem</tt> component, which is in
      charge of implementing memory management & input stream access, as
      well as <tt>ftinit</tt>, which is in charge of library initialization
      (i.e. implementing the <tt>FT_Init_FreeType()</tt> function).</p>
    </li>
    <li>
      <p>FreeType&nbsp;2 comes also with a set of <em>optional
      components</em>, which can be used either as a convenience for client
      applications (e.g. the <tt>ftglyph</tt> component, used to provide a
      simple API to manage glyph images independently of their internal
      representation), or to access format-specific features (e.g. the
      <tt>ftmm</tt> component used to access and manage Multiple Masters
      data in Type&nbsp;1 fonts).</p>
    </li>
    <li>
      <p>Finally, a module is capable of calling functions provided by
      another module.  This is very useful to share code and tables between
      several font driver modules (for example, the <tt>truetype</tt> and
      <tt>cff</tt> modules both use the routines provided by the
      <tt>sfnt</tt> module).</p>
    </li>
  </ul>

  <p>Hence, a more complete picture would be:</p>

  <center>
    <img src="detailed-design.png"
         width="390" height="429"
         alt="Detailed FreeType design">
  </center>

  <p>Please take note of the following important points:</p>

  <ul>
    <li>
      <p>An optional component can use either the high-level or base API.
      This is the case of <tt>ftglyph</tt> in the above picture.</p>
    </li>
    <li>
      <p>Some optional components can use module-specific interfaces ignored
      by the base layer.  In the above example, <tt>ftmm</tt> directly
      accesses the Type&nbsp;1 module to set/query data.</p>
    </li>
    <li>
      <p>A replaceable component can provide a function of the high-level
      API.  For example, <tt>ftinit</tt> provides
      <tt>FT_Init_FreeType()</tt> to client applications.</p>
    </li>
  </ul>

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